The karst ecosystem is one of the three most fragile ecosystems worldwide (Chen et al., 2020). In this ecosystem, surface water infiltrates underground, and subsurface runoff tends to exceed surface runoff. Southwest China is the most concentrated area of karst landforms (Zhang et al., 2017). The cultivated land is thin and easily disturbed by the environment, such as soil erosion, overcultivation, and excessive application of chemical fertilizers and pesticides. This situation leads to low productivity of cultivated land in karst areas, and the use rate of nitrogen fertilizer is as low as 30% (Ju, 2014; Yang et al., 2021). The main reason is the migration and transformation of nitrogen fertilizer in the soil (Wang et al., 2023). After nitrogen enters the soil, in addition to being absorbed by crops, some nitrogen remains in the soil as inorganic and organic nitrogen, and the vast majority is lost through various pathways, including leaching, runoff, and gas. Studies have shown that applying nitrogen fertilizers to the soil in karst areas has a low retention rate and a high nitrogen leaching rate (Zhang et al., 2021). This condition leads to the deterioration of soil structure and fertility (Bai et al., 2020), resulting in air, water resources, and agricultural non-point source pollution problems (Savci, 2012; Chandini et al., 2019). Therefore, it is crucial to study the characteristics of nitrogen fertilizer migration and transformation, adopt effective fertilizer management methods, improve the nitrogen fertilizer use rate, and reduce nutrient loss.

Many scholars have studied the effects of different soil types, soil moisture, soil pH, fertilization methods, and fertilizer properties on soil nitrogen transfer and transformation (Wallace et al., 2019; Dromantienė et al., 2020). For example, NH₄ ⁺-N produced by urea in paddy soil in an anaerobic soil layer can be protected from nitrification and denitrification. Urea application in deep soil can improve N absorption, and most NH₄ ⁺-N remains in the rice root zone (Wallace et al., 2019). The 10–20 mm precipitation levels were simulated in the 5–20°C soil temperature range. The NO₃ ⁻-N content in the 0–15 cm soil layer increased with soil temperature and rainfall when urea was applied. In contrast, the NH₄ ⁺-N content decreased. An effective way to improve the nitrogen use rate of crops is by combining fertilizers and inhibitors, which could delay nitrification and reduce nitrogen loss (Wallace et al., 2019). Still, nitrification inhibitors and urease inhibitors are generally used. Nitrification inhibitors inhibit the bacterial activity of nitrite in soil, inhibit nitrification, and delay the transformation of NH₄ ⁺-N to NO₃ ⁻-N, thus reducing nitrogen loss (Kim et al., 2012). Urea can be hydrolyzed by urease after it is applied to the soil. Urease inhibitors slowed down the hydrolysis rate of urea and reduced the volatilization and nitration of ammonium nitrogen by competing with urease active sites (Cantarella et al., 2018). Liu et al (Liu  et al., 2022). investigated the nitrification mechanism of different nitrification inhibitor types in red soil, rice soil, and tidal soil. They found that nitrification inhibitors were able to inhibit the conversion of NH₄ ⁺-N to NO₃ ⁻-N in both rice and tidal soils and that the inhibition effect of tidal soil was better than that of rice soil. In contrast, the inhibition effect of nitrification on red soil was not obvious. Increasing the concentration of oxalic acid in the rhizosphere of the soil in karst areas can strengthen the ability of plants to obtain nutrients in the karst ecosystem. Oxalic acid significantly accelerates the mineralization and availability of nitrogen and significantly increases the concentration of total inorganic nitrogen (Yuan et al., 2018). Soil improvement with oxalic acid and inhibitors (DMPP and NBPT) can reduce nitrogen loss and improve the nitrogen use rate.

When nitrogen fertilizers are applied to soil, fertilizer microsites are formed near the fertilizer, with nutrient concentrations significantly different from the soil overall. In the fertilizer microsites of tidal soils, most of the applied phosphorus was immobilized, mainly within 2 mm of the soil at the point of application (Du et al., 2012). In the fertilizer microsites of Paddy soil, water-soluble phosphorus, and effective phosphorus gradually decreased with increasing temperature (Chen et al., 2014). Soil types are mainly yellow-brown, yellow, and red soils in karst areas (Luo et al., 2023). Yellow-brown soil is a critical agricultural area, producing various food and cash crops. However, a study on the migration and transformation of nitrogen fertilizer in infertile microsites of yellow-brown soil in karst areas is needed. Yellow-brown soil was used as the study object in this experiment. The effect of nitrogen fertilizer combined with soil improver and nitrogen fertilizer synergist on nitrogen migration and the transformation level of soil in karst areas was studied through an indoor simulated soil column culture for 30 days. The migration law of nitrogen in the soil was clarified, providing theoretical guidance and a scientific basis for efficiently utilizing nitrogen in agricultural production.

Many studies showed that urease inhibitors and nitrification inhibitors could improve the use efficiency of nitrogen fertilizer and reduce losses and had the potential to alleviate nitrogen pollution in agricultural production (Giovannini et al., 2009). The results showed that the NH₄ ⁺-N and NO₃ ⁻-N contents in the soil column treated with oxalic acid and inhibitors were different in different water contents. It was easy to produce a negative charge on the surface of the soil colloid, NH₄ ⁺-N could be adsorbed by the soil colloid, and NO₃ ⁻-N could not be adsorbed and easily lost with water. The migration sites of NH₄ ⁺-N in A3 and A4 treatments were prolonged in ammonium bicarbonate treatment. This result indicated that ammonium bicarbonate combined with DMPP and NBPT could inhibit the activity of nitrifying bacteria and extend the fertilizer efficiency and migration distance of ammonium bicarbonate to some extent. There was no significant difference in soil NH₄ ⁺-N content during treatment after adding ammonium sulfate with oxalic acid (DMPP). According to WANG and TORRALBO et al (Torralbo et al., 2017; Wang et al., 2018), ammonium sulfate application could maintain the acidity of the rhizosphere. At the same time, DMPP had a short half-life in acidic soils and was most effective in suppressing nitrification in viscous soils with high pH values (Cui et al., 2021). Hence, adding DMPP to ammonium sulfate did not result in any significant changes in soil NH₄ ⁺-N content. The soil NH₄ ⁺-N content with ammonium chloride and 30% oxalic acid was higher than that with other nitrogen fertilizers in the C treatment; with the increase in water content, NH₄ ⁺-N content decreased, and there was no significant change after the addition of inhibitors. SOURI and VIEIRA MEGDA et al (Souri, 2010; Vieira Megda et al., 2014). found that in addition to nitrification inhibitors, chloride could also significantly inhibit nitrification by microorganisms in soil by increasing Cl⁻ content in the soil, and the inhibition was positively correlated with chloride concentration in soil. Therefore, the NH₄ ⁺-N content of ammonium chloride and the inhibitor did not increase significantly in the C treatment. In the D treatment, the NH₄ ⁺-N content of urea combined with DMPP increased at each point at 40% water content and flooded state. Although urea would rapidly hydrolyze to NH₄ ⁺-N after application in wet soil, NH₄ ⁺-N produced in the anaerobic soil layer was reduced by nitrification and denitrification due to anoxia (Siddique et al., 2019). Cantarella et al. (2018) found that NBPT showed high efficiency in delaying urea hydrolysis under anaerobic conditions. However, urea combined with NBPT had no significant effect on nitrification in the soil in this experiment. This result may be due to the influence of soil pH and organic carbon on soil urease and nitrification inhibitors in karst areas (Luchibia et al., 2020).

The differences in NH₄ ⁺-N content of soil treated with four nitrogen fertilizers may be attributed to the different rates of NH₄ ⁺-N provided for nitrification after hydrolysis of varying nitrogen fertilizers (Luchibia et al., 2020). Ammonium carbonate, ammonium sulfate, and ammonium chloride were all forms of nitrogen fertilizer containing ammonium ions (NH₄ ⁺) or ammonia (NH₃) (Yang et al., 2016). Ammonium carbonate was unstable and volatile, and it could evaporate even at room temperature. The nitrogen content of ammonium carbonate was lower than that of ammonium sulfate and ammonium chloride as it evaporates faster at high temperatures and with high moisture and better ventilation. As an amide nitrogen fertilizer, crops can only absorb and utilize urea after hydrolyzing into ammonium carbonate or ammonium bicarbonate (Wang, 2021). Therefore, its ammonium nitrogen content was lower than that of the first three fertilizers.

The NO₃ ⁻-N content in the soil was the highest under the combined application of DMPP. The NO₃ ⁻-N content of ammonium bicarbonate combined with oxalic acid and nitrification inhibitor was higher than that of other nitrogen treatments, and NO₃ ⁻-N content decreased with the increase of water content. Under the treatment of ammonium sulfate, ammonium chloride, and urea combined with DMPP, the NO₃ ⁻-N content in soil was higher than that of oxalic acid in each nitrogen fertilizer. The NO₃ ⁻-N accumulation in the soil was high when ammonium bicarbonate and ammonium sulfate with DMPP were applied in this experiment. At the same time, Yan et al (Yu et al., 2020) studied NO₃ ⁻-N content in soil. They found that the best fertilizer N forms of DMPP were ammonium sulfate and urea. This result may be because the effect of the nitrification inhibitor was affected by multiple factors, such as soil PH value and nitrogen application rate (Shi et al., 2016).

Ammonium bicarbonate combined with oxalic acid or inhibitors could prolong the horizontal migration distance of soil NH₄ ⁺-N in karst areas and, combined with inhibitors, increase the NH₄ ⁺-N content in the soil. Combining ammonium sulfate with oxalic acid or NBPT effectively increased the NH₄ ⁺-N content in the soil at 30% and 40% water content. Urea combined with DMPP increased NH₄ ⁺-N content in soil with increased water content. Ammonium chloride combined with inhibitor was not significant. Four nitrogen fertilizers combined with DMPP significantly increased the NO₃ ⁻-N content in the soil. Ammonium chloride combined with oxalic acid can make the percentage rate of NH₄ ⁺-N in soil reach 55.31%. The NO₃ ⁻-N percentage rate in soil with ammonium bicarbonate combined with DMPP reached 1.89%. The percentage rate of soluble inorganic nitrogen in the soil was 55.97% when ammonium chloride combined with oxalic acid was applied with a 30% water content.

In conclusion, soil NH₄ ⁺-N content was increased by ammonium bicarbonate combined with DMPP or NBPT, ammonium sulfate combined with oxalic acid or NBPT, and urea combined with DMPP. Four nitrogen fertilizers with DMPP increased NO₃ ⁻-N content in the soil. Therefore, it effectively improved the percentage rate of nitrogen fertilizers and the effective use of nitrogen fertilizers. Furthermore, the experiment results will be truthed in the field experiment, which has been conducted. This result will be presented next paper.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

WJ: Formal analysis, Investigation, Software, Supervision, Writing – original draft. CQ: Formal analysis, Investigation, Software, Supervision, Writing – original draft, Conceptualization, Data curation, Funding acquisition, Methodology, Project administration, Resources, Validation, Visualization, Writing – review & editing.